1. Field of the Invention
The invention relates to the Basic rate Interface (BRI) for the Integrated services Digital Network (ISDN). More particularly, the invention relates to a PBX switch which incorporates methods and apparatus for automatically detecting call appearance values for each Primary Directory Number (PDN) in an ISDN BRI.
2. Brief Description of the Prior Art
The Basic Rate Interface (BRI) of ISDN service consists of two B channels, each having a bandwidth of 64 kbits/s and one D channel having a bandwidth of 16 kbits/s. Generally, each of the B channels can be used to support one voice connection or one data connection, or the two B channels can be used together to support one 128 kbits/s data connection. The D channel is used for signaling between the customer equipment (terminal equipment or TE) and the central office in order to setup, manage, and tear down connections. Each B channel is assigned a directory number (DN), i.e. a ten digit telephone number.
Two signaling protocols are used with BRI ISDN in order to establish and maintain connections: The layer 3 Q.931 protocol and the LAPD (link access protocol for the D channel). Q.931 messages are carried within an LAPD frame on the D channel. A Q.931 message includes several octets specifying protocol discriminator, length of call reference value, call reference value, message type, etc. Some of the messages sent using the Q.931 protocol include SETUP, SETUP ACKNOWLEDGE, CALL PROCEEDING, CONNECT, CONNECT ACKNOWLEDGE, RELEASE, RELEASE COMPLETE, HOLD, and HOLD ACKNOWLEDGE.
The 1999 Version of the Beilcore National ISDN BRI Terminal Equipment (TE) Generic Guidelines (SR-4620, Issue 1, Dec. 1998 specifies guidelines for implementing many ISDN features. Among these features is a feature known as Call Appearance.
With Call Appearance, a single BRI can respond to up to 128 DNs. Moreover, customer equipment can be designated with up to 128 terminal endpoint identifiers (TEIs) each of which identifies a specific telephone or device connected to the BRI. Using Call Appearance and TEIs, each of up to 128 telephones or other devices connected to a single BRI can be provided with a separate DN. Further, each of the telephones connected to the single BRI can be configured as multiline phones responding to several DNs using electronic key telephone service (EKTS).
EKTS permits a number of illuminated keys on a telephone set to be addressed by the central office to indicate virtual additional lines. Those skilled in the art will appreciate that regardless of the number of Call Appearances, the single BRI only supports two active telephone calls at one time. More calls may be placed  on hold at the central office, using Q.931 signaling, but only two calls may be active at one time.
Separate Call Appearance values (0-63) are assigned to each B channel according to the service contract chosen by the customer. As mentioned above, two DNs are assigned to the BRI. This is the case when both B channels have a Call Appearance value of 0. These DNs are referred to as the primary directory numbers (PDNs). Each PDN may have a Call Appearance value other than 0 (up to 63) associated with it depending on the customer service contract. The call appearance value indicates the number of additional DNs associated with the B channel identified by the PDN.
As mentioned above, the Call Appearance values are in the range 0 to 63 for each B channel. The values used and the specific meaning of each Call Appearance value are defined during the ordering process in an agreement between the customer and the Central Office.
The different values are usually used to handle Central Office features (e.g. call waiting indication, voice mail message, call forwarding) and multiple calls for same B channel. In addition, choosing a special arrangement of Call Appearances and directory numbers allows a PBX to offer the equivalent of Direct Inward Dial capability over standard BRI lines.
Table 1 below illustrates an example of three BRI lines with multiple Call Appearances and shared multiple directory numbers. The main directory number for each B channel is italicized.
TABLE 1Circuit 1Circuit 2Circuit 3CAB1B2B1B2B1B201555-4001555-4001555-4001555-4001555-4001555-400102555-4002555-4002555-4002555-4002555-4002555-400203555-4003555-4003555-4003555-4003555-4003555-400304555-4004555-4004555-4004555-4004555-4004555-400405555-4005555-4005555-4005555-4005555-4005555-400506555-4006555-4006555-4006555-4006555-4006555-4006
Incoming calls for any of the six directory numbers are presented by the Central Office on all associated channels with the same Call Appearance value. In the PBX, each of the Call Appearances is assigned to a different station.
According to the state of the art, customer equipment must be manually configured with a service profile identifier (SPID) as well as the number of Call Appearances for each PDN. The correct configuration of BRI trunks and telephone equipment at the customer's premises requires specialized service engineers. The equipment must be configured for Call Appearance Call Handling (CACH) based on documentation provided by the central office. The available documentation is often not accurate or not up to date during the installation and (remote) configuration of the telephone equipment. A wrong configuration could leave trunk interfaces unusable by the customer.
The configuration of customer equipment is even more difficult when multiple BRIs are involved. For example, many PBX systems utilize multiple ISDN BRIs as an alternative to multiple individual DS1 or more expensive T1 connections. The Siemens Hicom 150 provides up to 16 BRIs through four interface cards, each of which supports four ISDN BRIs. Configuring a Hicom 150 could conceivably require configuring up to 2,048 Call Appearances (32*64).